The invention relates to voltage-source inverter power supply in general, and more particularly to a pulse-width modulated AC motor drive having such a power supply.
In any AC power supply it is necessary to ascertain the magnitude of the AC currents in each phase. This is done usually with current sensors. This is also the case with an AC motor drive.
The control circuit of an ac motor drive in general uses information with regard to the level of motor current mover in order to perform certain regulating functions under motor operation, such as: torque control, current limit, flux regulation, etc. Current information is usually obtained through current sensors inserted in series with the motor lines. At least two such sensors are required, which must be capable of delivering undistorted signals at very low frequencies, including DC. These sensors must also provide isolation between the control circuits and the power lines. Because of the low frequency requirements, transformers are not applicable. Because of the isolation requirements, direct sensing via shunts is not practical. As a result, sensing current as done in the prior art always necessitates circuit having complexity and it entails both a cost penalty and an impairment of the reliability of the system. Solutions have been proposed which consist in using either transducers based on Hall effect devices, or chopper-stabilized isolation amplifiers. It is known that either of these solutions adversely affect the cost of the controls and their reliability, as compared, for instance, to transformer-based current sensing.
In the case of a voltage-source inverter AC motor drive, a current sensor is also used in the DC link. This sensor provides information regarding the magnitude of the DC link current. This information is supplied to the control circuit for regulation and protection purposes with respect to the inverter, or at the front end where the DC source is. Such a DC current sensor has also bandwidth and isolation requirements, like the aforementioned AC current sensors, and the implementation in the prior art adpots solutions which are similar to those mentioned hereabove.